1. Field of the Invention
The present invention relates to a portable telephone terminal having an external adapter connected thereto through a short-range wireless link and in particular to communication control techniques for the portable telephone terminal and the external adapter.
2. Description of the Related Art
In recent years, a portable telephone device such as a cellular phone has become used not only as a telephone but also as a sophisticated information terminal having various additional functions, for example, browsing information on the Internet, e-mail transmission/reception and the like. However, voice communication is still of essential importance to such portable telephone devices.
To ensure ease in carrying, a portable telephone device becomes smaller in size. But smaller size makes a portable telephone device inconvenient to use because the distance between a microphone (transmitter) and a speaker (receiver) thereof is also made shorter.
One solution to such a problem is to employ a headset having an earphone and transmitter therein, which is connected to the portable telephone device by radio.
Japanese Patent Application Unexamined Publication No. 11-220765 discloses a combined mobile communication device. A mobile communication device is formed by combining a main unit with a radio transceiver, a voice input/output device having an earphone-microphone therein and other components, which are individual units connected to the main circuit unit by wireless interfaces.
Japanese Patent Application Unexamined Publication No. 2002-33824 discloses a portable communication device equipped with a radio transmitter for communication with a base station and a BlueTooth® transmitter for data communication directly with another portable communication device. In this conventional portable communication device, the remaining amount of battery is monitored and, when it drops below a predetermined threshold level, data transmission by the BlueTooth transmitter is terminated so as to avoid destroying important data to be transferred from a master to a slave.
However, in the case of a portable telephone device having a handset connected thereto by a short-range radio communication link, a problem arises when the communication quality of a radio channel to the handset has been deteriorated. When the quality of the short-range radio communication is reduced due to some reason, for example, a failure of the portable telephone device, the voice communication with the base station is also made difficult. In such a case, the base station is not expected to do channel disconnection processing or the like. It is necessary for the portable telephone device or the handset to handle such a problem.
The above conventional portable communication device equipped with the radio transmitter and the BlueTooth transmitter cannot handle such a problem of deteriorating the quality of short-range radio communication link because it looks at only the remaining amount of battery.